If a man makes a solemn promise to the Lord, or pledges to do something by swearing an oath, he must not break his promise. He must do everything he said he would.
If you foolishly swear to do something (whether good or bad, and in whatever way people impulsively swear an oath), even if you're unaware of it being wrong, when you eventually realize it, you are guilty.
Look! A messenger is coming over the mountains, bringing good news and proclaiming peace. Judah, celebrate your religious festivals and keep your vows, for never again will wicked enemies invade your land—they will be completely destroyed.
What a disaster is coming on those of you who say, ‘If you swear by the Temple that doesn't count, but if you swear by the gold of the Temple, then you have to keep your oath.’ What blind guides you are!
“Please don't listen to them, because they have planned to ambush him with more than forty men who have taken an oath not to eat or drink until they have killed him. They're ready right now, waiting for you to agree to the request.”
So Jephthah went with the elders of Gilead, and the people made him their leader and army commander. And Jephthah repeated all his conditions before the Lord at Mizpah.
When the two months were over, she returned to her father, and he did to her as he had promised, and she was a virgin. This is the origin of the custom in Israel